The present invention relates to a hydraulic circuit for driving a ram of hydraulic press.
In general, a hydraulic circuit using such as a 4-ports spool valve and a directional control valve has applied for driving a ram of a hydraulic press. Especially it has been made use of a servo valve for operating the ram with high speed.
In such the hydraulic circuit, a servo valve is provided in a line connecting a hydraulic cylinder and a variable displacement hydraulic pump. The hydraulic pump is driven by a motor for absorbing oil from a oil tank to store up the hydraulic oil into an accumulator. The hydraulic pump supplies hydraulic oil to a P-port of the servo valve. At the delivery side of the hydraulic pump, a pressure reducing valve is connected. The reducing output pressure of the pressure reducing valve is to be equal to the pilot pressure of the servo valve, whose A-port is connected with the upper chamber of the hydraulic cylinder, and whose B-port is connected with the lower chamber of the hydraulic cylinder, respectively. The T-port of the servo valve is linked with the oil tank. Supplying and filling up the upper chamber or the lower chamber of the hydraulic cylinder with the pressure oil, the ram fixed the rod of the hydraulic cylinder is to be moving upward or downward.
Considering the case of stamping out a work in the stamping machining process adopting this hydraulic circuit. When acting a load on the ram under the feed back control detecting a position of the ram, the opening rate of the servo valve is large so that in the instance of having stamped out a work the load acting on the ram is decreasing rapidly. Then the ram is running by inertia so that it is easy to give rise to an over-shoot. As a result, using this hydraulic circuit, there is a problem that this over-shoot causes to an increase of moving distance of the ram and a dead time when operating the ram with high speed.
Considering the case of the form machining process such as a drawing machining. In this machining, it is impossible to hold a constant pressure in the lower chamber of the hydraulic cylinder. As a result, there is a problem that it is difficult to assure the positioning control of the bottom dead center of the ram with high accuracy.
There is an essential problem that in order to drive the ram of the hydraulic press using a servo valve with high speed operation, it is necessary for a servo valve with high response and with large amount flow rate and a hydraulic pump with large amount flow rate, so that the energy efficiency becomes bad.
The present invention has as its purpose the solution of the above mentioned problems. It is the purpose of the present invention to offer a HYDRAULIC CIRCUIT FOR DRIVING A RAM OF HYDRAULIC PRESS which can control a hydraulic cylinder driving the ram of the hydraulic press with high speed and with good efficiency and which can operate with a heavy load.
The purpose of the present invention is to offer a hydraulic circuit for driving a ram of hydraulic press which is able to prevent the ram from running by inertia in the stamping machining process and which is able to assure the positioning accuracy of the bottom dead center of the ram and which it is easy to control using a control equipment such as a NC controller.